Autonomic nervous system activity is essential for maintaining circulatory and metabolic homeostatis. In order to study sympathetic nervous system function and its relationship to other neuroendocrine systems, it is necessary to measure neurotransmitter, hormonal, and peptide levels in response to various stimuli. The levels of norepinephrine, epinephrine, and dopamine and their metabolites in various body fluids reflect the activity of the neurones from which these neurotransmitters are released. Although plasma levels of norepinephrine reflect the responses of the peripheral sympathetic nervous system it is necessary to consider removal rates of the catecholamine. Measurement of urinary catecholamine metabolites and their stereospecific labeling pattern following administration of radiolabeled isomers of norepinephrine provides a means for investigating intraneuronal norepinephrine metabolism. Cerebrospinal fluid levels of monoamine metabolites can be used to assess central nervous system neurotransmitter metabolism. It is necessary to consider the origin of these metabolites to make appropriate corrections for valid interpretations of the data. These strategies have been used to study patients with neurogenic orthostatic hypotension and in other clinical situations in which adrenergic function is abnormal. Investigation of the effects of aging on autonomic nervous system function is in progress. A more thorough understanding of neurotransmitter metabolism in these clinical situations leads to more rational approaches to therapy.